Conventional multiple disc clutches and brakes employed in a drive train to transmit torque in an automatic transmission for an automotive vehicle or the like generally include a clutch disc pack consisting of a plurality of engagable clutch plates alternating with clutch separator plates interleaved therewith; the clutch plate surfaces having suitable friction linings, such as a resin-based, paper friction material, and the separator plates having generally smooth surfaces on the opposite faces thereof to engage the friction linings on the faces of the clutch plates. One set of plates has interior splines to engage a rotatable member and the other set of plates has exterior peripheral splines to engage either a second rotatable member or a stationary member to form the torsional coupling. Thus, one set of plates are splined to an encompassing clutch or brake drum while the other alternating set of plates are splined onto a central shaft or ring.
Prior efforts to improve the frictional characteristics of a clutch resided in the improvement of the friction lining for the clutch plates or discs, and substantially no attention has been paid to the frictional characteristics of the separator plates. One problem of the separator plates is in wear of the separator plate surfaces due to the frictional characteristics of the clutch plates. Less wear of the separator plates would provide more of a "steady state" rotating member and therefore consistent performance. There also is a need for higher friction to support current higher torque applications. More recently where the friction linings for the clutch plates have been formed of a paper-based friction material, the separator plates should not be formed of a material or have a surface texture which would cause undue wear upon the friction lining.
As the clutch plates have friction linings secured to the opposite faces thereof, the separator plates are conventionally finished by a tumbling process to have relatively smooth surfaces engaging with the friction lining material. To enhance the frictional characteristics of the steel plates when used in combination with the friction lined clutch plates, it has been suggested that the surface be altered to change the characteristics of the steel plates. Various surface preparation procedures were tried, such as sand blasting, shot peening and knurling, however, the resulting surface either did not enhance the coefficient of friction or caused undue wear and/or damage to the friction lining of the mating clutch disc faces. This, in turn would shorten the useful life of the clutch plates in the clutch pack for the automatic transmission.
Also, coating of the separator plates was attempted using a wear-resistant material, such as sputter coating of the steel plates with titanium nitride or similar metallic nitrides; materials known for their wear resistant properties for tool bits, saw blades, etc. Although a slight increase in the frictional characteristics may have been detected, this improvement was clearly offset by a noticeable increase in the wear on the friction linings. The present invention overcomes the wear problems for the friction material while increasing the coefficient of friction for the cooperating clutch discs and separator plates.